The present invention relates to a process of making synthetic board and boards produced therefrom wherein the final product i.e. the formed board has improved dimensional stability under varying moisture conditions.
The technologies of manufacturing wood-based composites have been continuously improved. It is no longer an imagination but a reality that wood-based composites can be produced stronger and stiffer than plywood, solid wood and laminated wood. The production rate has also been significantly increased through the advances in resin technologies. However, in many applications, wood-based composites are much inferior to plywood, solid wood and laminated wood due to lack of dimensional stability. Therefore it is not exaggerated to have a statement "the most severe drawback of wood-based composites is lack of dimensional stability".
For panel products, the mat is usually formed in such a way that the grain direction of furnish is generally parallel to the panel surfaces and the pressure direction is perpendicular thereto. The furnish is compressed in the thickness direction. Consequently, the thickness direction is the most unstable direction in wood-based panels.
The thickness swelling of wood based composite panels consists of reversible and irreversible swelling when the panels absorb water or moisture. The former is due to the hygroscopic nature of wood and the latter is due to the springback of compressed wood. The reversible swelling is normally less than the solid wood because the hygroscopicity of wood is reduced by heat during hot pressing. The irreversible swelling is the main cause of instability of wood-based composites. Therefore, the irreversible swelling must be radically reduced in order to improve the dimensional stability of wood-based composites drastically.
Irreversible swelling results from the release of pent-up internal stresses in the composite absorption of water or moisture. Therefore it is reasonable to believe that highly stable composites can be produced if the composite is made in such a way that internal stresses are minimized during pressing.
Thickness swelling of wood-based composite board is undesirable particularly where such boards are used in exterior applications and other applications where uncontrolled moisture conditions exist.
The dimensional stability of a composite board or panel is normally determined by measuring the thickness swelling of the panel following controlled exposure to moisture. Conventional wood-based composite boards or panels can experience a thickness swelling ranging from 10 to 25 percent of the panel's thickness following a horizontal 24 hour cold water soak and which can range from 20 to 40 percent if subjected to a vertical 24 hour cold water soak. When subjecting a conventional panel to a 2 hour boiling period followed by a 1 hour cold water soak, thickness swelling in the range of 50 to 60 percent can be anticipated. As a result, the use of conventional composite boards and panels as a construction material is limited to installations and environments where the moisture conditions are controlled or anticipated in advance so as to take preventative steps. As a consequence, wood-based composites are regarded as undesirable for exterior applications and particularly ground contact applications because of differential dimensional changes between the wet and dry portions of the material below and above the ground. The moisture and moisture cycling effect experienced by composite panels subjected to variations in humidity or exposure to water also contribute to the break-down or degradation of the panel rendering it unfit as a construction material for the purpose intended. Indeed, building contractors are reluctant to use wood-based composite panels as a flooring or sub-flooring because the edges of a panel can exhibit greater thickness swelling than the panel's central portion and thus detracts from a substantially planer abutment joint with neighboring panels.
The dimensional stability i.e. thickness change of waferboard or other composites can be improved by increasing the resin content, press time or press temperature. Increases in resin content increase the production costs significantly and therefore is undesirable. Increasing press time also is undesirable from a production cost point of view and therefore not considered effective. Increase of press temperature is effective but results in a fire hazard and therefore again is undesirable.
A principle object of the present invention is to provide a process for producing highly stable wood-based composite board without resorting to high pressure or high temperature treatments and without increasing resin content or resorting to special high-cost resin binders.
Another object of the present invention is to provide a process for producing highly stable and bond durable products and products produced by such process which can be further treated with preservatives, fire retardants or other chemicals without causing significant damage to strength and excessive thickness swelling.